Physical dependence on phenobarbital was produced in female Sprague-Dawley rats. A change in estrus behavior was noted in that cycling stopped in the estrus portion of the cycle in animals undergoing treatment with phenobarbital. Phenobarbital blood levels were determined. Brain metabolism in withdrawing animals was studied using the 2-deoxyglucose technique. 14C-deoxyglucose uptake in the brains of phenobarbital withdrawing rats appeared grossly similar to that seen with alcohol withdrawal in that columns and ovoid regions of increased uptake were present in the cortex and cerebellum, respectively. The lateral geniculate appears banded with the dorsal portion darker than the ventral portion. The autoradiographs of the phenobarbital withdrawing rats also appeared darker than controls. Densitometric measurements to quantitatively compare alcohol and phenobarbital withdrawing rats are in progress.